In recent years, a more interventionist style of management has been trialled on the Great Barrier Reef in the form of the Targeted Control Program for outbreaks of the coral-eating crown-of-thorns starfish. Incorporating recommendations from applied scientific research into starfish control activities has significantly improved the efficiency and effectiveness with which key reef sites can be defended. There is growing acceptance that further ecosystem interventions will be necessary in a changing climate. To maximise the chances of successful outcomes, it will be important that these defensive interventions are coordinated and based on the best available science and engineering.
A number of factors determine thermal tolerance of corals (Schloepf et al. 2015) including thermal history, species composition, reduced fitness due to other pressures and local hydrodynamics that influence water movements and temperature. It is well-accepted that reefs that are near upwellings, or experience cooler currents or wind mixing, tend to experience less severe bleaching, and those with few other pressures tend to recover better. Observations suggest that under doldrum conditions even very small increases in water movement around corals could advect heat and metabolic by-products away from colonies (Nakamura et al. 2003), facilitate oxygen transport and photosynthetic efficiency (Finelli et al. 2006), and deliver carbon as a heterotrophic food source (Lesser et al. 1994).
Temperature data from AIMS loggers at 2.5 m (red), 6.0 m (green) and 12.0 m (blue) at the tourism pontoon at Agincourt Reef during the 2017 mass bleaching event (1 Jan-9 Apr 2017). The first signs of early bleaching (stage 1) were observed near the pontoon on 4 February (Doug Baird, pers comm). These data suggest that augmenting natural flushing & mixing processes could feasibly provide as much as 1.3 °C difference in temperature for near-surface corals in the lead-up to and during times of peak stress.
First-pass assessments indicate that there are a range of potential coral stress countermeasures that mimic these natural phenomena and could feasibly reduce localised coral stress in small areas. Previous studies have investigated the merits of measures such as shade cloth and sprinkler systems at very small scales (generally in aquaria) (eg Smith & Birkeland 2007). Existing technology developed for destratification of large water bodies could be readily repurposed for deployment at the scales discussed in this proposal. Marine engineers have estimated that the power required to completely turn over the water at such a site (approx. 250 x 250 x 10 m) twice a day is in the order of ~3000 W, which is less than that generated by a normal household solar installation.
Finelli, C.M. et al. (2006) Water flow influences oxygen transport and photosynthetic efficiency in corals. Coral Reefs 25: 47-57
Lesser MP et al. (1994) Effects of morphology and water motion on carbon delivery and productivity in the reef coral Pocillopora damicornis: diffusion barriers, inorganic carbon limitation and biochemical plasticity. J Exp Mar Biol Ecol 178: 153-179
Nakamura, T., Yamasaki, H., van Woesik, R. (2003) Water flow facilitates recovery from bleaching in the coral Stylophora pistillata. Marine Ecology Progress Series, 256, 287-291
Schoepf, V., Stat, M., Falter, J. L., McCulloch, M. T. (2015) Limits to the thermal tolerance of corals adapted to a highly fluctuating, naturally extreme temperature environment. Scientific reports, 5, 17639
Smith LW & Birkeland C (2007) Effects of intermittent flow and irradiance level on back reef Porites corals at elevated seawater temperatures. J Exp Mar Biol Ecol 341:282-294